PROJECT ABSTRACT Swarming motility, exhibited by many motile species of bacteria, has been implicated in the rapid invasion of hosts during urinary tract infections (UTIs). Annually, UTIs result in several thousand deaths in the US alone and represent a significant load on the public healthcare system. Swarming motility is substrate-associated and is driven by bacterial flagellar motors that rotate extracellular, helical filaments to generate thrust on the cell- body. Although chemotaxis is not required for swarming, the functioning of a molecular switch that enables reversals in the direction of motor-rotation is indispensable. The switch is activated by CheY-P, an intracellular response-regulator that is regulated by the chemotaxis network. Upon CheY-P-binding, cooperative interactions within the multi-subunit switch-complex drive concerted transitions from counterclockwise (CCW) to clockwise (CW) conformations with increasing likelihood, resulting in changes in the direction of rotation. Our recent results indicate that flagellar motors sense mechanical forces, arising from contact with solid substrates, and that leads to the inhibition of switching. In a short time the motor adapts to these forces and recovers the ability to reverse directions. However, the molecular underpinnings responsible for adaptation remain unclear. Thus, there is a critical need to determine how the switch adapts to mechanical stimuli to promote swarming. Without such knowledge, the potential to capitalize on antivirulence strategies as therapeutic approaches to combat swarming-mediated host-invasion and antibiotic resistance will likely remain limited. Our long-term goal is to contribute toward the development of new clinically useful antivirulence strategies that target bacterial swarming and colonization. Our overall objective in this application is to determine the molecular mechanisms whereby the switch adapts perfectly to mechanical signals and promotes swarming. Our central hypothesis is that motor-mechanosensing (sensing of mechanical signals) results in the tuning of ultra-sensitivity through the modulation of allosteric and cooperative interactions within the switch. The rationale for the proposed work is that a determination of the mechanism of mechanical control of ultra-sensitivity is likely to provide a conceptual framework for the development of strategies to interfere with switch adaptation, and to mitigate swarming. At the completion of the proposed research, it is our expectation to have quantitatively explained the mechanisms underlying switch-adaptation and modulation of ultra-sensitivity by mechanical forces. Results are expected to have an important positive impact because a detailed understanding of switching near substrates will provide a strong foundation for novel substrate-design in biomedical devices, including catheters, which will target the motor-switch to inhibit swarming.